Rare earth-cobalt alloys are peculiarly suitable for the production of permanent magnets since many such alloys exhibit extremely large uniaxial magnetic crystal anisotropy.
Prior art permanent magnets manufactured from rare earth-cobalt alloys contain a rare earth component which consists essentially of a first component of 10-25 atomic percent of Y, Sc, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb or Lu, wherein some of the indicated elements may be used alone or at least two or even all of them in mixture, and a second component which amounts to 75-90 atomic percent of the alloy and which consists of cobalt alone or of cobalt in mixture with one or several of the elements manganese, iron, nickel and copper.
For the purposes of this invention, the term "rare earth-cobalt alloys" is therefore primarily directed to alloys of the indicated kind, to wit, alloys which consist essentially of 10-25 atomic percent of at least one of the elements Y, Sc, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb or Lu, and about 75-90 atomic percent of cobalt, or cobalt in mixture with one or several of the elements manganese, iron, nickel and/or copper. The above-mentioned elements manganese, iron, nickel and/or copper may replace at most 10 atomic percent of said cobalt.
Rare earth-cobalt alloys of the indicated kind and a method for processing them into permanent magnets have been disclosed in the U.S. Pat. Nos. 3,424,578 and 3,540,945 to which attention is directed.
It is well known by those skilled in this art that pulverized rare earth-cobalt alloys have the important drawback that their coercive force has a tendency to decrease after some time. If permanent magnets composed of such alloys are thus stored for prolonged periods, the decrease in the coercive force is very considerable. This tendency, of course, negatively affects the technical applicability of such alloy powders and permanent magnets made therefrom.
With a view to overcoming this serious drawback and to increasing the coercive force field strength, it has been proposed to treat the alloy powder in a nickel bath or to galvanically coat them with one of several other metals, including tin. The results obtained thereby have, however, not been satisfactory.
It has also been suggested to improve the coercive force characteristics of such rare earth-cobalt alloy powders by subjecting them to an acid treatment. While it is true that such treatments initially increase the coercive force of alloy powders obtained by grinding, it has also been established that such treatments do not prevent the aging phenomena which ultimately result in a reduction of the coercive force. In fact, they appear to accelerate them.